The hierarchical establishment of tendon structure and function during development, maturation and healing is dependent upon collagen I assembly into fibrils and higher order structures. The regulatory fibril-forming collagens XI and V are essential in the regulation of fibril assembly and fiber organization, despite being quantitatively minor components. Collagen XI is expressed during development, but is virtually absent in mature tendons, while collagen V is expressed throughout development, maturation, and in mature tendon. During the injury response there is a transient increase in collagen XI and sustained increase in collagen V expression. Importantly, the clinical phenotype in patients with mutations in COL11A1 with Stickler's Syndrome and COL5A1 with classic Ehlers Danlos Syndrome includes joint laxity involving tendons and ligaments. Also, collagen XI and V genes are linked to tendinopathy while collagen V genes are linked to ACL rupture in women. These findings support critical regulatory mechanisms for these collagens in the establishment of tendon structure and function, re-establishment after injury, as well as alterations associated with pathologies supporting our general hypothesis that collagen XI- and synergistic collagen XI/V-mediated mechanisms are necessary for establishing tendon structure/function and that these mechanisms are recapitulated after injury. Our specific aims are to: (Aim 1) Define the mechanism(s) involving collagen XI interactions and synergistic collagen XI/V interactions regulating the hierarchical assembly of the tendon required for function. The hypotheses are that establishment of initial tendon structure and function requires interactions involving collagen XI while continued development and maturation require coordinate collagen XI and V interactions; (Aim 2) Elucidate the regulatory mechanism(s) involving collagen XI and/or synergistic roles of collagens XI/V in the tendon response to injury. The hypothesis is that regulatory mechanisms involved in the reacquisition of tendon structure and function require coordinate collagen XI and XI/V expression. Specifically, altering collagen XI or XI/V expression will cause an impairment of repair including fibril assembly, wound matrix organization, and integration of new matrix into the surrounding unwounded matrix influencing structural organization of the tendon and its mechanical properties. Collagen XI will influence the early stages of injury response while synergistic collagen XI/V interactions will have a broad impact in all stages. Our multidisciplinary approach will utilize novel mouse models to modulate collagen XI and XI/V expression followed by a definition of the effects on the structural, macro-scale mechanical, fibril mechanical, and compositional properties. These studies will define tendon-specific regulatory mechanisms involving collagens XI and XI/V, providing a mechanistic understanding of the acquisition of tendon structure and function including its re-establishment in response to injury. Further, the data will provide a critical foundation for developing future therapeutic interventions for modulating these critical collagens in disease states or following injury. Page1